1. Field of the Invention
The present invention relates to a communications system using a network and in particular relates to a packet conversion device and a packet conversion method for communicating by converting a multi-cast data transfer packet to a uni-cast data packet that can be used in one-to-one communications in a network when the network does not support a multi-address calling for transmitting a message from a transmitter to a plurality of receivers at one time, that is, multi-cast communications.
2. Description of the Related Art
For example, recently a demand for a live relay and a conference type multi-cast communications service on the Internet has been increasing. The first prior art of such a multi-cast communication system is described with reference to FIG. 1. In the first prior art, for example, when an IP (Internet protocol) packet is inputted to the Internet, a method of attaching an IP header to the top of the packet is adopted to transfer the IP packet to multiple clients as a multi-cast data packet on the Internet.
For example, in the case of a live relay, multi-cast data packets are always transmitted from a server 50 for distributing the data to a network. Each of a plurality of clients requesting the live relay asks a network side for the relay of multi-cast data packets always transmitted from the server 50, and the multi-cast data packets are relayed. In such a case, multi-cast data packets transmitted from the server 50 are inputted to a router 52 for controlling packet transfer on the Internet via a gateway 51. In this case, a packet transmitted from the server 50 stores both an IP header A and a UDP (user datagram protocol) header B. However, when the packet is inputted from the gateway 51 to the router 52, an IP header X for distributing the packet to a plurality of clients is attached to the top of the packet, and the packet is inputted to the network.
FIG. 2 shows the second prior art of a multi-cast communications system. This technology is disclosed in the following reference.
Japanese Patent Laid-open No. 10-242962: Multi-cast Gateway Communication System and Method on the Internet
In FIG. 2, it is assumed that a network already supports multi-cast communications and it is a terminal on a client side requesting the reception of multi-cast data that does not support multi-cast communications. In this case, for example, an IP multi-cast data packet that is transmitted from a server 55 and is transferred on an IP multicast-supported network 56 is copied as much as required by a gateway on the client side, and is generally relayed to a plurality of terminals on the side of a client 58 as an IP uni-cast data packet.
In the first prior art of an IP header adding system described with reference to FIG. 1, since an IP header must be attached to a packet transferred on a network, a part of the packet must be deleted in the gateway 57 when data equivalent to the maximum packet length of the Ethernet are attempted to be transmitted, and the data corresponding to the maximum packet length of the Ethernet cannot be transmitted, which is a problem. If there is a failure on the network, the analysis of the failure is difficult since the packet format is different from an ordinary one, which is another problem.
In the second prior art described with reference to FIG. 2, a part that does not support multi-cast communications is limited to a terminal in a receiving side. However, since software for supporting multi-cast communications is sold on the consumer market, the terminal can easily support multi-cast communications. It is a case where a network side, such as a router, etc., does not support multi-cast communications that matters. Although it can be said that the Internet basically supports multi-cast communications, the number of IP addresses for multi-cast communications are limited. For example, a virtual public network (VPN), where LANs between the headquarters of an enterprise and each of the plants are connected using the Internet, cannot support a large scale of multi-cast data communications, which is a problem.
It is an object of the present invention to provide both a packet conversion device and a packet conversion method for converting a packet header at the entrance/exit of a network to transfer a multi-cast data packet as a uni-cast data packet on the Internet in light of the problems described above.
The packet conversion device of the present invention is provided at the entrance/exit of a network and comprises a header conversion means for rewriting the header of a multi-cast data transfer packet and inputting the packet to the network as a uni-cast data transfer packet and/or rewriting the header of the uni-cast data transfer packet received from the network and outputting the packet as a multi-cast data packet.
According to the present invention, the header of a packet transmitted as multi-cast data from a terminal is converted to the header of uni-cast data corresponding to the multi-cast and is transmitted. Therefore, even if the network does not support multi-cast communications, the transfer of multi-cast data can be implemented by the same process as that of the transfer of uni-cast data.
Although a header is converted, new information is not added. Therefore, there is no change in packet length. Accordingly, the packet can be handled in the same way as another packet.